A pulse oximeter estimates the degree of oxygen saturation of the hemoglobin in the arterial blood (SaO.sub.2). Modern instruments use optical techniques in conjunction with a noninvasive sensor to achieve the estimate. The sensor of the oximeter radiates a section of well perfused tissue with at least two wavelengths of light. The light contacts hemoglobin contained in red blood cells. A certain amount of light is absorbed by the hemoglobin. The amount of light absorbed depends on the wavelength of light and whether the hemoglobin is oxygenated. By knowing the wavelength of light being used and the relative amount of light being absorbed, it is possible to estimate SaO.sub.2. The amount of light being absorbed is typically determined in pulse oximeters by comparing the amount of return light detected at systole and at diastole. The difference between these two readings is determined for each wavelength. This difference is assumed to be the light absorbed by the hemoglobin in the oxygenated blood.
In previous systems, taking these light measurements involves using a photodetector which is sensitive to both wavelengths of incident light, to provide an electrical signal. This signal is then filtered to remove the non-changing or DC components. The remaining pulsatile component is then amplified, filtered to remove noise, and typically converted to digital format for processing. The difference between the light measurements is generally computed in software, although it would be possible to use peak detectors and difference circuits to perform the calculation in analog.
Available commercial pulse oximeters place the sensor's light source on the opposite side of an appendage from the detector. In this manner, all light received by the detector has been transmitted through the appendage. There are several disadvantages to placing an oximetry sensor on an appendage. For example, if the patient goes into shock, the extremities are the first to lose blood flow. If blood flow is lost, the oximeter cannot estimate SaO.sub.2.
Recently, a pulse oximeter sensor as shown in FIG. 3 has been developed that uses reflected rather than transmitted light to estimate SaO.sub.2. The sensor 15 for this device has light sources 18 and 20 of different wavelengths positioned on the same side of the tissue 21 as the detector 22. With this arrangement, the detector receives only that light that is scattered back (reflected) to the detector 22. The advantage of the reflectance oximeter is that it is not limited to being located on appendages.
A major problem encountered in the implementation of a reflectance oximeter is obtaining accurate readings of the signals returned from the tissue. The difficulty lies in detecting and accurately measuring the extremely low amplitude pulsatile signals which are riding on top of very large levels of non-pulsatile or DC signals. For reflectance measurements, the ratio of pulsatile to DC components of the return signals can be anywhere from 5% to less than 1%.
The algorithms used to determine the oxygen saturation from reflectance signals require as inputs accurate measurements of not only the pulsatile components, but also the DC components. Each of these signals are generally required to be measured within an accuracy of at least 1%. In addition, the exact relationship of the pulsatile to DC signals in terms of gain, time, and frequency response must be controlled and known. Furthermore, the low ratio of pulsatile to DC components in the reflectance signal makes the system much more sensitive to errors introduced to the signal by the signal conditioning and conversion.
Thus, there remains a strong need in the art for an improved oximeter system which avoids erroneous blood oxygen saturation measurements as a result of errors introduced when separating the DC and pulsatile components of the oximeter sensor signal prior to microprocessor analysis.